The present invention relates to radio frequency amplifiers. Specifically, the physical layout of wiring interconnections used to evenly distribute the total current and power of an RF power amplifier having multiple bipolar amplifying transistors operating in a parallel circuit configuration is disclosed.
Heterojunction bipolar transistors often provide more efficient RF power amplification than other semiconductor devices in integrated circuit form. Extremely high power efficiency can be obtained because of the high power density and high threshold, breakdown voltage of the transistors. For high power designs, a multitude of devices are used in some form of parallel structure to distribute the total power over a sufficiently large surface, such that excessive heat does not accumulate to degrade the performance or reliability of the devices.
During normal operation, the total amplifier current is equally distributed through the numerous transistors and excessive heat and other problems do not occur. However, if the parallel transistors are slightly mismatched, one transistor will operate at a higher temperature than the others and draw a larger amount of current. Since the combined current carried by the parallel devices of the power amplifier is more than enough to destroy a single transistor, the possibility exists for a transistor to experience xe2x80x9cthermal runaway.xe2x80x9d Thermal runaway results when one device fails and the remaining operational devices must bear the amplification burden without the failed device. To maintain the amplification, the remaining operational devices each must bear a larger portion of the combined current flowing through the power amplifier. Due to the increased current each remaining transistor bears, these transistors become increasingly heated. If an transistor becomes too hot, it too will fail, thereby causing a chain reaction failure of the other transistors comprising the power amplifier.
Unfortunately, even small differences in device characteristics or physical placement can cause an imbalance of heating between the individual devices. Any bipolar device that is connected in parallel with other similar devices and that becomes hotter than its neighbors will tend to draw more current, thus heating itself more. The heating compounds itself and the result is a thermal runaway phenomenon which will destroy the device and the integrated circuit (IC) that incorporates the device.
A prior art RF power amplifier, output stage comprised of multiple heterojunction bipolar transistor elements, connected in a parallel circuit configuration, is illustrated in the schematic of FIG. 1. The multiple transistor transistors, labeled Q1A-Q1N, form the output stage of the power amplifier and can be imagined as effectively forming a more powerful unitary transistor, Q1. Prior art solutions typically employ additional circuit elements to achieve thermal stability. These additional elements are placed in series with the output device. Since these additional elements contain a current impedance component, the power efficiency of the circuit is degraded. Therefore, it is highly desirable to convert as much power as is possible from the DC power supply into RF power at the output of the circuit. U.S. Pat. No. 5,629,648 is illustrative of prior art designs used to balance the load through the individual elements of the large effective output transistor Q1. The insertion of resistors RB1-RBN into the circuit of FIG. 1, to balance the power load through the transistors Q1A-Q1N, reduces the power amplifier efficiency.
The present invention recognizes that the physical layout of the power amplifier, output stage, of FIG. 1, is an important design consideration. The parasitic inductances of the wire interconnections used to make all of the necessary connections to create the simplified schematic of FIG. 1, produce appreciable impedances at the operational RF frequencies. If ignored, these impedances can an undesirable mismatch in the instantaneous currents at the leads of transistors Q1A-Q1N and induce a thermal runaway situation.
The solution to the above-described problem is obtained by configuring the physical layout of the schematic to minimize the impedances of the conductive interconnections at the operational RF frequencies. Additionally, it is necessary to provide substantially the same transient current and voltage waveforms to the base inputs of each output transistor so that the waveform at each input is substantially in phase with the waveform at all of the other parallelly connected base inputs.
The present invention provides a side fed amplifier comprising a plurality of transistors connected in parallel such that the base, emitter, and collector leads of each transistor are electrically connected to the base, emitter, and collector leads, respectively, of all other transistors. A common, physical point interconnects the power amplifier current source and the base leads of every transistor. The transistors are spacially arranged such that the impedance between the common physical point and the base lead of any one transistor is substantially equivalent to the impedance between the common point and the base lead of any other transistor within the power amplifier.